1. Field of the Invention
The present invention relates to a reagent for coagulation test used in examination of the extrinsic clotting system of blood and a method for manufacturing the reagent.
2. Description of the Related Art
Thromboplastin is a complex between a protein called tissue factor and phospholipid. Thromboplastin is involved in blood coagulation. Hence, thromboplastin has been utilized in various coagulation tests. For example, a reagent for thrombo-test for comprehensively examining the coagulation ability of factors II, VII and X (or factors II, VII, IX and X) in blood contains bovine brain tissue thromboplastin, fibrinogen, factor V, calcium and phospholipid.
Bovine brain tissue thromboplastin as a major component in the reagent for thrombo-test is a complex having a phospholipid added to a protein called tissue factor. The tissue factor is a protein consisting of 257 amino acids in total as shown in FIG. 3 in Yuko Takayenoki et al.: “cDNA and amino acid sequences of bovine tissue factor”, Biochemical and Biophysical Research Communications, Vol. 181, 1991, pages 1145-1150. The tissue factor consists of an extracellular domain (soluble domain) of from the N-terminal to amino acid 213, a transmembrane domain of from amino acid 214 to amino acid 236 and an intracellular domain of from amino acid 237 to amino acid 257.
Conventionally, thromboplastin having such constitution has been produced by extracting thromboplastin from a bovine cerebrum as a starting material with acetone powder or physiological saline.
A bovine cerebrum used as a starting material is one site affected highly with bovine spongiform encephalopathy (BSE). Hence, there is recently demand for a manufacturing method without using a bovine cerebrum as a starting material.
There are reports on manufacturing, by utilizing genetic engineering techniques, of a recombinant tissue factor having an activity as a composition of a measurement reagent. Examples of such reports include WO 93/07492, WO 98/48283, and a literature of Cheryl L Brucatol et al. (“Expression of recombinant rabbit tissue factor in Pichia pstoris, and its application in a prothrombin time reagent”, Protein Expression and Purification, Vol. 26, 2002, pages 386-393).
Specifically, in WO 93/07492 supra, a recombinant human tissue factor is expressed in Escherichia coli. Then, the recombinant human tissue factor is purified by affinity chromatography with an immobilized monoclonal antibody to human tissue factor. Then, the purified recombinant human tissue factor is applied to a prothrombin reagent.
In the literature of Cheryl L Brucatol et al., a recombinant rabbit tissue factor (rTF) is expressed in yeasts. Then, a histidine tag is used to purify rTF, and the resulting rTF is applied to a prothrombin time reagent. In this literature, it is reported that the expressed rabbit tissue factor is a full-length tissue factor consisting of an intracellular domain, a transmembrane domain and an extracellular domain.
In WO 98/48283, a full-length recombinant rabbit tissue factor consisting of an extracellular domain, a lipid-bound domain and an intracellular domain is expressed in yeasts. Then, a histidine tag specifically enriching the recombinant tissue factor is utilized to purify the recombinant rabbit tissue factor. Then, the purified rabbit tissue factor is applied to a prothrombin time reagent.
As described above, a recombinant tissue factor purified by chromatography etc. is used in order to prevent the reduction in the coagulation activity of a reagent, or the reduction in measurement accuracy, caused by contamination of the reagent with host-derived impurities. For purification of the recombinant tissue factor, however, it is necessary to construct a system for efficiently separating and removing host-derived impurities. When the recombinant tissue factor is produced in a large amount, the step of purifying it requires considerable time, labor, cost etc., and thus there is a problem in productivity of the recombinant tissue factor.
In the purification method using affinity chromatography with a specific antibody, for example, use of the specific antibody causes higher costs in mass production.
For the recombinant rabbit tissue factor, for example, the purification method using affinity chromatography with a histidine tag has been practically used as a purification method suited to mass production. However, the linking of a histidine tag to the recombinant tissue factor may exert an influence on coagulation activity, depending on the type of host or the type of tissue factor. In this purification method, therefore, the histidine tag is cut off after purification. Addition of this step of cutting the tag off leads to an increase in production costs.
Depending on the steric structure of tissue factor, the tissue factor may not be successfully purified even by chromatography with a histidine tag.
From the foregoing, a production system capable of satisfying productivity with respect to costs and quantity of production without influencing coagulation activity should be constructed for production of the tissue factor by genetic engineering techniques. It is then desired to provide a reagent for measuring clotting time using a tissue factor obtained by the constructed production system.